Technique to forecast where the next big quake will start

Scientists have a new method for studying faults that could improve earthquake forecasts, shedding light on where quakes start, how they spread, and where the biggest impacts might be.

UCR geologist and lead author Nic Barth at the Alpine Fault. The Australian Plate is to the left. (Jesse Kearse/Kyoto University)

A paper in the journal Geology describes the method, which helps determine the origins and directions of past earthquake ruptures - information valuable to modeling future earthquake scenarios on major faults.

By studying subtle curved scratches left on the fault plane after an earthquake, similar to the tire marks left after a drag race, scientists can determine the direction that earthquakes came from to that location.

"Fault planes accumulate these curved scratch marks, which until now we didn't know to look for or how to interpret," explained UC Riverside geologist and paper first author Nic Barth.

Curved scratches have been observed on fault surfaces following several historic ruptures including the 2019 Ridgecrest earthquakes in California. Computer modeling was used to confirm that the shape of the curvature indicates the direction the earthquake came from.

This study is the first to demonstrate that this method can be applied to fingerprint the locations of prehistoric earthquakes. It can be applied to faults worldwide, helping to forecast the effects of possible future earthquakes and improve hazard assessments globally.

"The scratches indicate the direction and origin of a past earthquake, potentially giving us clues about where a future quake might start and where it will go. This is key for California, where anticipating the direction of a quake on faults like San Andreas or San Jacinto could mean a more accurate forecast of its impact," Barth said.

Geologist Tim Little measuring curved scratches on the Alpine Fault. (Nic Barth/UCR)

Where an earthquake starts and where it goes can have a big influence on the intensity of shaking and the amount of time before people feel it. For example, scientists have shown that a large earthquake originating on the San Andreas fault near the Salton Sea that propagates to the north will direct more damaging energy into the Los Angeles region than a closer San Andreas earthquake that travels away from LA.

More optimistically, such an earthquake that starts further away could allow cellular alert systems to give Angelenos a warning of about a minute before the shaking arrives, which could save lives.

New Zealand's Alpine Fault is known for its regular timing of large earthquakes, which makes it a more straightforward choice for studying fault behavior. The fault is known to rupture at almost metronomic intervals of about 250 years.

This study provides two valuable insights for the Alpine Fault. First, that the most recent quake in 1717 traveled from south to north, a scenario that has been modeled to produce much greater shaking to populated areas. Second, it establishes that large earthquakes can start on both ends of the fault, which was not previously known.

Examples of curved scratches documented in this study. (Nic Barth/UCR)

"We can now take the techniques and expertise we have developed on the Alpine Fault to examine faults in the rest of the world. Because there is a high probability of a large earthquake occurring in Southern California in the near-term, looking for these curved marks on the San Andreas fault is an obvious goal," Barth said.

Ultimately, Barth and his team hope that earthquake scientists around the world will start applying this new technique to unravel the past history of their faults. Barth is particularly enthusiastic about applying this technique across California's fault network, including the notorious San Andreas Fault, to improve predictions and preparedness for one of the most earthquake-prone regions in the United States.

"There is no doubt that this new knowledge will enhance our understanding and modeling of earthquake behavior in California and globally," he said.

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